Electrical stimulation has been used for beneficial effect in medicine, most notably for cardiac pacemakers, to sustain the rhythm of the heart. The nerves of the body respond to electrical impulses. In addition to the heart, the nerves of the body have been targets for artificial electrical stimulation. Unlike cardiac stimulation in which a single pulse can trigger a contraction of the entire heart, neurological stimulation relies upon many pulses to recruit various nerve fibers.
Stimulation of the vagus nerve is used to treat epilepsy and depression while a number of other medical conditions including eating disorders, dementia and obesity are under investigation. The human has left and right vagus nerves which descend from the base of the brain into the thorax. Current systems for stimulating the vagus nerve, so called vagal nerve stimulators, employ an electrode implanted in the cervical region of the neck. An exemplary electrode is described by Schwartz et al in U.S. Pat. No. 3,421,511 herein, incorporated in its entirety, by reference. The electrode is connected to a lead, an insulated conductor that extends between the electrode and an implanted pulse generator. An exemplary electrical stimulator for the vagus nerve is described by Zabara in U.S. Pat. No. 5,025,807 herein, incorporated in its entirety, by reference. For controlling or preventing epileptic seizures, the pulse generator generates electrical pulses having a frequency of between 30 and 300 cycles per second (Herz), a pulse duration of between 0.3 and 1 millisecond and a constant current of between 1 and 20 milliamperes. The generator is implanted in the body. Electrode leads pass from the generator through a subcutaneous tunnel and terminate in an electrode patch on the vagus nerve.
When stimulating the vagus nerve, the recurrent laryngeal nerve may also be stimulated as it is part of the vagus nerve. Some applications take advantage of this effect such as that described by Goldfarb in U.S. Pat. No. 5,111,814, incorporated herein in its entirety, by reference. However, for medical applications where laryngeal stimulation is unintended, it may have an undesired effect. As described by Baker in U.S. Pat. No. 5,205,285, incorporated herein in its entirety by reference, stimulation of the vagus nerve can cause undesirable modulation of the voice. The effect on the voice can include alteration of the voice, coughing and hoarseness. While reducing the strength of the stimulation may alleviate the undesired effect, the beneficial medical effect may also be lost. To achieve therapeutic efficacy, sufficient stimulation strength must be delivered. Unfortunately, the undesired effects of laryngeal stimulation can reduce the amount of stimulation which can be tolerated by the patient.
Baker (U.S. Pat. No. 5,205,285) discloses an apparatus to suppress the stimulation while the patient is speaking including a speech sensor and discriminator. The Baker apparatus provides selective suppression of the stimulation while the patient is speaking. The suppression of nerve stimulation is ceased after a preset time interval regardless of continued detection of speech.
Ben Ezra et al, in US pub. No. 2005/0065553 A1 and incorporated herein in its entirety, by reference, describes applications of vagal stimulation in which stimulation parameters are varied with detected phases of respiration.
If the adverse effects of the vagus nerve stimulation are avoided, a higher output of the implantable stimulator device might be tolerated by the patient and the therapeutic effect increased. Without an apparatus to mitigate the adverse effects of vagus nerve stimulation the stimulator output strength may be limited by the adverse effect to the level that the patent can tolerate.